Patent GB-483238-A (GB)
Title: Improvements in or relating to electric protective arrangements
IPC: H02H3/28
Filed: 1936-10-13
Abstract: 483,238. Protective cut-out systems. REYROLLE & CO., Ltd., A., and LEYBURN, H. Oct. 13, 1936, No. 27769. [Class 38 (v)] A protective arrangement of the Merz-Price, earth-leakage type, having core-balance transformers or line-current transformers with paralleled secondaries, has means for causing a, test-current, during normal service, to flow in at least a part of the residual-current circuit, an alarm relay or other indicator arranged to show when the normal test-current flow is disturbed, thereby giving an indication (without operation of the trip-relays) as to the stability of the system with external faults or its response to network to which a number of external feeder or other circuits are connected. Fig. 1 shows a simple bus-bar A connected to feeders B in which core-balancer transformers are arranged, with their secondaries D interconnected by a residual-current circuit and in parallel with a trip-relay E and an indicator or alarm-relay H requiring less operating-current than relay E. Each transformer has an auxiliary winding F, which windings are connected in series to an A.C. source G for continuously or intermittently testing the residual circuit. For three feeders, one winding F has twice the number of turns of the others and is connected so that the current therein is in the opposite direction to that in the other windings, whereby the testing- current is normally confined to the windings D and their interconnections but does not traverse the relay H. A breakdown in those windings or interconnections would therefore produce an unbalance and operate relay H, and would provide a test as to the stability of the gear on through-earth faults, since the unbalance-fault current does not traverse the trip-relay E. In order to ensure also the correctness of operation with internal faults, the unbalance currents of which traverse the relay E, it is necessary also to test the circuit containing that relay. Accordingly a change-over switch J is arranged periodically to supply only the two small windings F with testing-current which flows in and operates relay H if the circuit is sound. A suitable switch for performing the two tests has one " off," and two " on," positions in which latter the stability to external faults and response to internal faults are respectively tested. Where two bus-bar sections such as A are interconnected by a section-switch, each section may have an arrangement similar to that of Fig. 1, with the addition, however, to each arrangement of a transformer in the other section so that the section-switch is overlapped, Fig. 2 (not shown). Alternatively, where the section-switch is not so overlapped, an arrangement similar to that of Fig. 1 is provided for testing each section independently, and a switch M operated by the section-switch K is provided for connecting the two arrangements in parallel for testing the protective gear as a whole, Fig. 3. The windings F of the two sections are all connected in series so that normally there is a balance in the testing- currents in the residual-circuits of the two arrangements, but in order to include the auxiliary switch M itself in the sphere of testing, further switches N operated by the closure of the section-switch K, alter two of the testing- windings F to produce unbalance in the two arrangements, which unbalances are equal and opposite and so circulate testing - current through the auxiliary switch. If the latter fails, the unbalance flows through the alarmrelays. In order to avoid operation of both trip-relays E to cut out both bus-bar sections on internal fault, the relays operate their tripcontacts with a slight time-lag, but act to trip the section-switch instantaneously, so opening the auxiliary switch and allowing the healthy trip-relay to reset. In Fig. 4 (not shown), selective isolation of one of two faulty sections is effected by a balanced differential relay, arranged in the residual circuit between the two protective gear so as to be re-operated by the testing-current circulating therein, but to be operated by out-of-balance fault-current in the protected section. Fig. 5 shows a duplicate bus-bar arrangement in which the feeders are adapted to be connected over selectors R which control auxiliary switches S for connecting the transformers D to the appropriate bus-wire P<1> or P<2>. The latter are connected to one another over the coils of a differential relay Q, the midpoint of which coils is connected to the common bus-wire P over relays E, H. This arrangement avoids the difficulties arising where such relays are duplicated for each bus-wire P<1>, P<2>. The relay Q is balanced so as to be unoperated by any circulating testing-current that may flow through it. In order, however, to obtain balance of the testing-current regardless of the distribution of feeders among the bus-bars, the testing-current can be obtained from a line current transformer in each feeder and is therefore proportional to the feeder-current, Fig. 6 (not shown). In Fig. 7 (not shown), a duplicate bus-bar system is equipped with parallelconnected current transformers in each feeder, one end of the transformers of each feeder being connected to an auxiliary switch controlling a pair of bus-wires as in Fig. 5. One transformer in each group has in series with it the primary of an auxiliary transformer, having a secondary connected across the two bus-wires with its mid-point connected to the residual connection to the auxiliary switch. A tertiary winding is also provided and all the tertiaries are seriesconnected to an alarm relay. The arrangement is such that the auxiliary switch of each feeder short-circuits part of each secondary, so wiping out the flux in that transformer, but a fault in that switch removes the short-circuit, so allowing the tertiary to be energized and the relay to be operated. In Fig. 8 (not shown), the auxiliary windings F are connected in series in one arm of a Wheatstone-bridge, the balance of which is upset when the impedance of a winding F is modified by failure of the testing- circuit. The' Specification refers also to the deriving of the testing-current from one transformer having a different turns-ratio from that of the others in the group, and to changing the direction of current-flow in an auxiliary winding to disturb the testing-current balance.